This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Many important pathogens of humans and animals belong to the positive-sense, single-stranded RNA viruses. Enzymes such as RNA polymerase and the chymotrypsin-like cysteine proteases are absolutely required for viral replication. Therefore, these enzymes and their precursors are ideal targets for the design of antiviral drugs. We and others have determined crystal structures of a number of viral enzymes from this group, including some with bound inhibitors (for example, HCV, SARS and picorna-viruses). This work is valuable for the understanding of viral evolution, enzymatic mechanisms and the development of anti viral drugs. We are pursuing a project to develop different and more effective inhibitors of several viral enzymes.